In electrical or electronic systems, it is often desirable to design the input impedance of an electrical load (or the output impedance of a source of electrical energy) to maximize the power transfer and/or minimize reflections from the load. Maximum power transfer is typically obtained when the load impedance is equal to the complex conjugate of the source impedance. In contrast, minimum reflection can typically be achieved when the load impedance is equal to the source impedance.
Current radio frequency (RF) or high-frequency (HF) front end systems typically still comprise, at the transmitter end, a power amplifier (PA) for amplifying the signal to the required level, a filter (typically a harmonic filter), a power detector, and an antenna switch that performs a switching between the transmission bands, the reception bands, as well as between transmitter operation and receiver operation. Thereafter the signal is typically forwarded via an antenna impedance matching network to the antenna.
This antenna impedance matching is designed that, averaged over all use cases, frequencies and operating modes, as well as over their respective probabilities, an optimum is achieved. As can readily be seen, the optimum is only reached very seldom, because the frequency spectrum of the mobile communications frequencies constantly becomes broader and also the antenna itself provides very different matching for all frequencies and environmental conditions that may occur.
Moreover, in order to address all possible matching points, an inductive matching component as well as a capacitive matching component is needed. In order to reach all regions of the Smith chart, typically at least three components are needed. Due to the fact that combinations exist which would lead to very unfavorable component values, typically up to six reactive components are employed in order to obtain more flexibility. But also these matching networks may suffer from the fact that they achieve a high quality factor for only a selected choice of the component values and/or at a specific frequency. When the impedance matching network is set to another operating point, a significant degradation of quality factor can typically be observed.